1) Field of the Invention
The present invention relates to a method for issuing a command from a target side to an initiator side between a disk array apparatus having a target function only and an information processing apparatus having an initiator function only, a disk array apparatus and an information processing apparatus to which the method is applied, and a data management system including a disk array apparatus having a target function only and an information processing apparatus having an initiator function only to which the method is applied.
2) Description of the Related Art
A conventional disk array apparatus (redundant arrays of inexpensive disks: RAID) can access data massively stored in an external storage unit connected to a host computer in a high speed, with an improved reliability by providing a redundancy of the data at the time of an error occurrence (see, for example, Japanese Patent Application Raid-Open Publication No. 2004-164675). In general, the disk array apparatuses are classified into five levels, RAID1 to RAID5, according to speed of accessing data and level of the redundancy. FIG. 9 is a block diagram of the conventional disk array apparatus. The disk array apparatus includes a plurality of magnetic disk devices 201a, 201b, 201c, . . . , 201x, and 201y, which are constituted with hard disk devices, and a disk-array control unit 202 that performs a control of the magnetic disk devices 201a to 201y. 
Each of the magnetic disk devices 201a to 201y is used for different purposes corresponding to the level of the RAID. The purposes can be storing of data, mirroring the data stored in the magnetic disk device, storing parity data created for data stored in a magnetic disk device, etc. The disk-array control unit 202 includes a channel adapter 203 that is connected to a host bus adapter (HBA in FIG. 9) 211 of a host computer 210, and performs an interface control for the host computer 210, a disk array controller 204 is connected to the magnetic disk devices 201a to 201y, and performs a variety of controls when reading and writing data, and device adapters 205a, 205b, 205c, . . . , 205x, and 205y that perform a control of each of the magnetic disk devices 201a to 201y based on an instruction from the disk array controller 204 when reading and writing data.
The channel adapter 203 and the host bus adapter 211 are, for example, American National Standard Institute (ANSI)-standard small computer system interface (SCSI) devices, and some of them include a SCSI device (the host bus adapter 211) of the host computer 210 having a function of issuing a command (hereinafter, “an initiator function”) only and a SCSI device (the channel adapter 203) of the disk array apparatus having a function of receiving and executing a command (hereinafter, “a target function”) only. In this type of disk array apparatus, the host computer 210 having a SCSI device with the initiator function only performs a communication with a disk array apparatus having a SCSI device with the target function only, based on a SCSI standard, to perform a bidirectional data communication. For example, when the channel adapter 203 receives a read command from the host bus adapter 211, the disk array apparatus only performs a read process with the magnetic disk devices 201a to 201y via the device adapters 205a to 205y, and returns a result of the read process to the host bus adapter 211 (see, for example, Japanese Patent Application Laid-Open Publication No. H9-167132). In this case, only when the host computer having the initiator function only issued a command, the disk array apparatus having the target function only can perform only a response to the command.
However, these days, the storage units to be connected to the host computers are increasingly required to have larger capacity and higher access speed. Data management systems that fulfill such a demand are known. Such a data management system includes a hierarchy management unit that performs a hierarchy management for a disk array apparatus, a large-capacity storage unit such as an optical disk, data to be stored in the disk array apparatus, and data to be stored in the large-capacity storage unit. This type of data management system makes an access from a host computer to data smooth by performing a hierarchy management in which, for example, frequently used data from among the data stored in the large-capacity storage unit is disposed in the disk array apparatus. In this type of data management system, a bidirectional communication for transmitting and receiving a command is performed between the disk array apparatus and the hierarchy management unit, because a process such as deleting or writing of data is necessary.
However, it is the reality that disk array apparatuses having the target function only are widely used so far. With this type of disk array apparatus, it is not possible for the disk array apparatus to transmit a message to a host computer having the initiator function or a hierarchy management unit in a data management system. For this reason, the conventional disk array apparatus having the target function only cannot be used in the data management system in which a bidirectional data communication is frequently carried out between the disk array apparatus and the hierarchy management unit. Therefore, it is necessary to use a disk array apparatus including a channel adapter, a host computer, and a hierarchy management unit having both the initiator function and the target function. In other words, device adapters must be changed such that both the channel adapter of the disk array apparatus and a host bus adapter of the hierarchy management unit or the host computer have both the initiator function and the target function.